Neuronal demyelination is a deleterious condition characterized by a reduction of myelin in the nervous system. Vital to both of the central (CNS) and peripheral (PNS) nervous system, myelin encases the axons of neurons and forms an insulating layer known as the myelin sheath. The presence of the myelin sheath enhances the speed and integrity of nerve signal in form of electric potential propagating down the neural axon. The loss of myelin sheath produces significant impairment in sensory, motor and other types of functioning as nerve signals reach their targets either too slowly, asynchronously (for example, when some axons in a nerve conduct faster than others), intermittently (for example, when conduction is impaired only at high frequencies), or not at all.
Neuronal tissue generally comprises neurons and supporting glial cells. Glial cells outnumber neurons by about ten to one in the mammalian brain. Glial cells may be divided into four types: astrocytes, oligodendrocytes, Schwann cells and microglial cells. The myelin sheath is formed by the plasma membrane, or plasmalemma, of glial cells—oligodendrocytes in the CNS, and Schwann cells in the PNS. During the active phase of myelination, each oligodendrocyte in the CNS typically produce as much as approximately 5000 μm2 of myelin surface area per day and approximately 105 myelin protein molecules per minute (Pfeiffer et al. (1993) Trends Cell Biol. 3: 191-197). Myelinating oligodendrocytes have been identified at demyelinated lesions, indicating that demyelinated axons may be repaired with the newly synthesized myelin.
Neuronal demyelination is manifested in a large number of hereditary and acquired disorders of the CNS and PNS. These disorders include, for example, Multiple Sclerosis (MS), Progressive Multifocal Leukoencephalopathy (PML), Encephalomyelitis, Central Pontine Myelolysis (CPM), Anti-MAG Disease, Leukodystrophies: Adrenoleukodystrophy (ALD), Alexander's Disease, Canavan Disease, Krabbe Disease, Metachromatic Leukodystrophy (MLD), Pelizaeus-Merzbacher Disease, Refsum Disease, Cockayne Syndrome, Van der Knapp Syndrome, and Zellweger Syndrome, Guillain-Barre Syndrome (GBS), chronic inflammatory demyelinating polyneuropathy (CIDP), and multifocual motor neuropathy (MMN). For the vast majority of these disorders, there are no cures and few effective therapies.
During Parkinsons's disease (paralysis agitans or shaking palsy) cells of the brain appear to deteriorate for unknown reasons. However, a role for inflammatory reactions has been postulated to play a role in the pathogenesis of Parkinson's. Parkinson's is a disorder of the brain characterized by shaking and difficulty with walking, movement, and coordination. The disease affects approximately 2 out of 1,000 people, and most often develops after age 50. It affects both men and women and is one of the most common neurologic disorders of the elderly. Parkinson's disease is caused by progressive deterioration of the nerve cells of the part of the brain that controls muscle movement (the basal ganglia and the extrapyramidal area).
In addition to the loss of muscle control, some people with Parkinson's disease become severely depressed. Although early loss of mental capacities is uncommon, with severe Parkinson's the person may exhibit overall mental deterioration (including dementia, hallucinations, and so on). Dementia can also be a side effect of some of the medications used to treat the disorder.
Amyotrophic Lateral Sclerosis (ALS) is a rapidly progressive, invariably fatal, disorder causing loss of nervous control of voluntary muscles because of destruction of nerve cells in the brain and spinal cord resulting in loss of the use and control of muscles. The nerves controlling these muscles shrink and disappear, which results in loss of muscle tissue due to the lack of nervous stimulation. Muscle strength and coordination decreases, beginning with the voluntary muscles (e.g., those under conscious control). The extent of loss of muscle control continues to progress, and more muscle groups become involved. There may be a loss of nervous stimulation to semi-voluntary muscles, such as the muscles that control breathing and swallowing. Eventually, all muscles under voluntary control are affected, and patients lose their strength and the ability to move their arms, legs, and body.
Motor neurons located in the brain, brainstem, and spinal cord serve as controlling units and vital communication links between the nervous system and the voluntary muscles of the body. Messages from motor neurons in the brain (upper motor neurons) are transmitted to motor neurons in the spinal cord (lower motor neurons) and from them to particular muscles. In ALS, both the upper motor neurons and the lower motor neurons degenerate or die, ceasing to send messages to muscles. Unable to function, the muscles gradually weaken, waste away (atrophy), and twitch (fasciculations). Eventually, the ability of the brain to start and control voluntary movement is lost. The cause is unknown.
MS is the leading cause of nontraumatic CNS morbidity in young adults. The young age of onset and progressive nature of the disease imposes an enormous economic and social burden on society. Acute exacerbations in typical relapsing-remitting MS are the manifestation of acute and focal inflammation and demyelination in the CNS and have long been considered the primary pathology of MS. These events are the target of currently approved therapeutic agents. However, correlations of T2 inflammatory signal on magnetic resonance (MR) images and disease progression are weak as are the clinical characteristics during the relapsing-remitting (RR) phase and subsequent progression of disability. Furthermore, once irreversible disability is reached, the progression to further disability is not affected by relapses, including those occurring before or after the onset of irreversible injury.
In addition to permanent neurological disability due to axonal loss, inflammatory demyelination plays a role in MS pathogenesis. In contrast to inflammation, axonal loss typically correlates with T1 black holes, decreased N-acetyl aspartate (NAA) on magnetic resonance spectroscopy (MRS) and the degree of spinal cord atrophy, which can correlate with clinical disability in patients. These changes have been noted in patients as early as six months after diagnosis, but in most patients the chronic, and perhaps global, axonal injury breaches a clinical threshold at the onset of the secondary progressive phase of the disease.
During the acute inflammatory stage of the disease (clinically defined as relapsing/remitting MS, or RRMS), inflammatory mediators likely contribute to axonal injury. Associations have been made between the number of CD8+ T cells and the extent of axonal damage and animal models tend to support this implicating a CD8-MHC class I pathway of axon destruction (Rivera-Quinones et al., (1998) Nat. Med. 4:187-193). Further support comes from pathology studies in which activated CD8 T cells containing cytotoxic granules polarized toward the demyelinated axons suggests direct CD8+ T cell toxicity. Macrophages and microglial cells have been found in close proximity to degenerating axons. These cell types play a role in the homeostatic mechanism of removing debris from the CNS, however they also release inflammatory mediators including proteases, cytokines and free radicals such as nitrous oxide (NO). Finally, antibodies and complement may also play a role in axon damage during acute inflammation. Levels of anti-ganglioside antibodies were found to be significantly higher in primary progressive MS (PPMS) than in secondary progressive or RRMS and axons exposed to complement after demyelination may activate the complement cascade directly.
The relationship between inflammation and neuron loss in MS has not been fully delineated. There is a need to establish a model of oligodendrocyte loss and subsequent demyelination that does not rely on the induction of inflammation to specific CNS antigens systemically or necessitate the use of a potent system adjuvant. Such a model that utilizes antigens and inflammation typically fails to recapitulate demyelination and neuronal loss identified in MS patients.
An accurate animal model of axonal transection and neuronal loss should mimic the pathological features identified in MS brain specimens. This includes the identification of transected axons, transected dendrites and neuronal apoptosis in acute cortical lesions. The acute event should also result in measurable impaired neurotransmission that is restored, to varying degrees, by the redistribution of Na+ channels as has been identified in MS pathology specimens. Chronic lesions should demonstrate a variable degree of remyelination and smoldering persistent axonal loss should be evident. Finally, neuron loss in the animal model should be identified in regions anatomically distinct and temporally distinct from original demyelinating lesions mimicking the effect of the disease on NAWM. These features would provide an important and accurate depiction of the effects on neurons identified in pathology specimens from MS patients.
The delineation of the precise molecular mechanism and pathogenesis of neuropathy and in particular neuronal demyelination, has been hampered by the continued lack of effective animal models. Thus, there remains a pressing need for composition and methods to effect a robust screen for therapeutics directed to neuronal disorders.